PROJECT SUMMARY/ABSTRACT The overall goals of this proposal are to investigate the mechanisms of bladder injury (hemorrhagic cystitis) associated with BK polyomavirus (BKPyV) infection in children and young adults undergoing bone marrow transplantation (allogeneic hematopoietic cell transplantation, HSCT). BKPyV-associated hemorrhagic cystitis is a complication for up to 25% of allogeneic HSCT recipients. Hemorrhagic cystitis leads to patient morbidity from painful urination, prolonged hospitalizations, increased blood transfusion requirements, and invasive procedures when urinary obstruction is present. Each episode of hemorrhagic cystitis accounts for 10 additional hospital days and costs $70,000. In its most severe presentation, hemorrhagic cystitis may be associated with a higher risk of death. BKPyV replication in the urine can be detected in 80% of HSCT recipients, but not all of these patients will develop hemorrhagic cystitis. There are no antiviral therapies with proven efficacy against BKPyV and current treatment strategies for hemorrhagic cystitis are limited to supportive care. A major barrier to developing and testing novel therapies is our lack of understanding the mechanisms of bladder injury in patients with BKPyV replication. This also prevents the identification of the patients at highest risk for bladder injury. The central hypothesis of this proposal is that the risk of hemorrhagic cystitis after HSCT is a function of both BKPyV subtype and the host response to the virus. To test our hypothesis, we will study two separate prospective observational cohorts: an existing group of 193 children and young adults who received an allogeneic HSCT at Cincinnati Children?s Hospital Medical Center or the Children?s Hospital of Philadelphia with already banked urine samples and an independent validation cohort of 200 children and young adults undergoing allogeneic HSCT to be enrolled during the award period at these same two centers. The project?s specific aims are: 1) to test whether the population or within-person BKPyV diversity is different between patients with and without hemorrhagic cystitis after HSCT and to enroll the validation cohort; and 2) to test whether host cellular responses or specific gene polymorphisms are different between subjects with and without hemorrhagic cystitis after HSCT using proteomics and transcriptomics approaches. This study?s results are expected to increase our understanding of the viral and host mechanisms of bladder injury after HSCT. The data from this application are critical for identifying the patients at highest risk of hemorrhagic cystitis who would benefit from cellular therapies and other targeted, future prevention or treatment strategies.